Water or Oil?

A blacksmith wishes to cool his hot piece of steel as rapidly as possible. He has a bucket of ice-water and a bucket of oil (at room temperature). Which bucket should he dump his steel into?

Answer

He should put the steel into the oil as the liquid will lower the temperature quicker. The water will actually boil as it touches the metal, but that gas will insulate the hot steel from the rest of the cooling water.Hide

Comments

It's like when you get a frying pan heated up you can have a sizzling drop of water on it and it lasts a long time (again the insulating gas). I'm not sure how to put this extra comment into my teaser without putting it in this way.

Cool teaser...I actually got it, too! It was kind of easy though, because the ice-cold water would be too obvious of an answer, so I just said the oil anyway without really knowing why he would choose it. I did have a rough idea though. Very nice!

I've heard of people putting a wet finger in molten lead without any problems. I also read about a guy who placed some liquid nitrogen on his tongue with no problems. The nitrogen evaporates instantly and creates a cushion of air in between his tongue and the liquid.

As an engineer, I'm not sure that the answer is correct. But I may be wrong. It is true that the water vapor will insulate but the bubbles will quickly rise to the surface of the water and allow the water to stay in contact with the metal. The example where people dip their finger in water and then in molten lead without getting burnt only works because they only dip their finger in the molten lead for a fraction of a second. Other cases where the water vapor insulates is when the vapor can not escape easily. The latent heat required to vaporize water is huge, which makes it a great cooling medium. This is probably why water is still used to cool hot objects.

I disagree with this teaser. I too am an engineer (mechanical) and the process you're referring to is "quenching". Assuming ample amounts of each liquid (so that all of the water/oil doesn't burn off, etc.), the water at the same temperature, and especially at a lower temperature, will cool the steel more rapidly. Water has a higher specific heat (~4 kJ/kg*K) compared to a typical oil (~2 kJ/kg*K), and water has a higher thermal conductivity (~.6 W/m*K) compared to a typical oil (~.1-.2 W/m*K). This means water can absorb more heat for a given amount and can absorb it more quickly than oil. True, the steam from the boiling water will insulate the steel to some degree, but not enough to negate these other properties. The reason you've seen steel workers quench steel in oil is because it provides a more uniform cooling of the steel, whereas water (possibly due to the steam/bubbles forming) may cause softer deposits to form during quenching. I would refer to this website which states unequivocally that water cools more rapidly than oil for steel quenching:
http://www.key-to-steel.com/Articles/Art12.htm
Note the two paragraphs immediately below Figure 1.